The state of stress in rock plays an important role in the behaviour of the ground in response to the creation of underground openings such as found in mining operations or civil engineering projects. For the design of underground openings it is therefore essential that the pre-excavation in situ stress be either measured or estimated. The inability to measure or estimate satisfactorily the state of stress in many circumstances is one of the major impediments to the utility of many of the theoretical and mathematical models which have come into use for the design of underground excavations.
Stress is an intangible quantity which cannot be measured directly. It is only the manifestation of stress which is measured and used to estimate the stress. The most common methods employ stress relief or compensation techniques which result in strains which can be measured. By knowing the mechanical properties such as the modulus of elasticity and the Poisson's ratio of the rock in which the measurements were carried out one can deduce the in situ stress.
In most mining and civil engineering applications the in situ stress is measured with overcoring methods. These methods require the installation of a strain measuring device in a borehole and the subsequent drilling of an oversized hole over the existing hole to obtain an annulus which is stress relieved. The resultant strain is measured with the device inside the annulus. The stress regime can then be calculated from the strains measured as a result of the relaxation of the rock.
There are several different overcoring procedures and devices all of which have in common that they require the presence of a diamond drill throughout the testing procedure and, with the exception of the USBM gauge, the measuring devices cannot be recovered. As a consequence, these determinations are very costly and time consuming and can rarely be carried out on a routine basis.
Against this background a novel instrument was recently developed at James Cook University, Australia, which does not rely on overcoring methods and is fully recoverable. A full description of the method and associated apparatus is given by H. Bock et al in the Proceedings of the 4th Australian & New Zealand Conference on Geomechanics in Perth, Australia, 1984. The novel device is now manufactured and marketed by Interfels, Germany as the James Cook/Interfels Type 096 Borehole Slotter (hereinafter referred to as the Slotter). This is the instrument upon which the above invention is based and it is therefore described in more detail.
The Slotter works on the (St. Venant) principle that, when a crack or slot is created in the wall of a stressed borehole, virtually total stress relief will occur immediately adjacent and normal to the crack or slot. This results in a deformation of the rock which is controlled by the state of stress the rock was prior to the creation of the crack or slot as well as the mechanical properties of the rock material.
The Slotter is designed to cut, by means of a diamond saw blade, an axial slot into the sidewall of a borehole and to measure the tangential deformation of the rock caused by the resultant relaxation. The strain measurement is done with an integrated, specifically designed recoverable strain sensor. Repeated slots are cut around the circumference of the borehole wall and the deformation results are combined. To arrive at the two dimensional stress field normal to the axis of the borehole, the measured deformations, expressed in microstrains, which are a function of the stress acting perpendicular to the respective slots, and the physical properties such as the modulus of elasticity and the Poisson's Ratio of the rock mass, are used for a close form mathematical solution. To determine the three dimensional stress field in the rock mass, similar measurements are carried out in two additional, non-coplanar, non-coangular boreholes, and the results of three boreholes are combined by regression analysis.
Although the slotter is in many respects more efficient and cost effective than the overcoring methods, it has the disadvantage of needing three separate boreholes to determine the three dimensional state of stress in a rock mass. In view of the cost of drilling the required "H" size (96-106 mm) diamond drill holes, which can be as high as $5,000 per hole, considerable savings could be realized if the number of boreholes required could be reduced. In addition, the time needed to carry out measurements could also be reduced substantially if the procedures would not have to be repeated in three different boreholes. Lastly, it is difficult to combine the results of three independent, diverging boreholes into a single three dimensional stress tensor, especially when the rock is anisotropic and structured, which may cause significant differences in local stress orientations and magnitudes.
It is therefore the object of the present invention to provide a method and an apparatus, based on the slotting principle, which would allow the determination of the three dimensional stress regime in rock i) in a single borehole, ii) without the requirement of an on-site diamond drill and iii) with a device that can be recovered and reused.